A lighting circuit (ballast) for supplying a power in a stable manner is required for turning ON a discharge lamp such as a metal halide lamp to be used in a headlamp for a vehicle. For example, a discharge lamp lighting circuit disclosed in Japanese patent document JP-A-2001-6894 comprises an H bridge circuit, and an AC power is supplied from the H bridge circuit to a discharge lamp.
The discharge lamp lighting circuit also controls to turn ON the discharge lamp. More specifically, the discharge lamp lighting circuit controls a no-load output voltage (OCV: Open Circuit Voltage) before turning ON the discharge lamp and applies a high voltage pulse to the discharge lamp to turn ON the discharge lamp, and then carries out a transition to a stationary lighting state while reducing the transient input power.
In the discharge lamp lighting circuit having the structure described in the JP-A-2001-6894, power is stored in a capacitor having a large capacity and a current is supplied from the capacitor to the discharge lamp immediately after the lighting operation so that the discharge lamp can be turned ON in a stable manner. In the discharge lamp lighting circuit having a series resonant circuit, however, the capacitor sometimes is not provided in order to reduce a size of the circuit. Accordingly, an arc is not grown in the discharge lamp, resulting in a failure in the lighting operation after the high voltage pulse is applied. In the case in which the lighting operation fails, the discharge lamp lighting circuit needs to execute the lighting control again. For this reason, it is important to determine whether the lighting operation for the discharge lamp is successful or not.
Conventionally, it has been determined whether the lighting operation is successful or not depending on the presence of a current flowing to the discharge lamp (which will be hereinafter referred to as a lamp current). More specifically, the determination is based on whether or not a lamp current value exceeds a predetermined threshold. However, the lamp current obtained immediately after the lighting operation is relatively low, and a long time is required for executing the lighting control again if the determination about the lighting operation is delayed until the lamp current value is increased so as to be detectable. In the case in which the discharge lamp is used in a headlamp for a vehicle, it is preferable that the time taken from the ON operation of a power supply to the lighting operation be as short as possible.
In view of the foregoing problems, it would be useful to have a discharge lamp lighting circuit capable of shortening the time required for executing a lighting control again.